In UMTS WCDMA there is a mechanism defined for Random Access transmission in the uplink. The User equipment UE (namely the base station) transmits a randomly-selected preamble signal characterised by:
Signature sequence (i.e. bit sequence)
Scrambling code,
Sub-channel (i.e. timing of the access slot in the frame)
If the base station receives the signature it acknowledges it on the Acquisition Indicator Channel (AICH) with a corresponding signature. If the UE receives a positive acknowledgement it transmits a message part on a RACH data channel.
If the UE does not receive an acknowledgement (or a negative acknowledgement indicated by an inverted signature on the AICH) it can make another attempt. The probability of collision depends on the number of Signature sequences, Scrambling codes, Access slots from which the UEs make their selection for the randomly-selected preamble signal.
Currently in 3GPP it is proposed that UEs can be configured to use a new mechanism, which is similar to RACH. For the purpose of this invention, the main difference is that the data transmission which follows the preamble transmission uses a high-speed uplink packet data channel, known as Enhanced Dedicated Channel (E-DCH), instead of a single short message associated directly with the RACH preamble. Unlike the RACH message, the E-DCH is characterised by power control, rate control, hybrid ARQ and the ability to transmit high data rates for an indefinite length of time, but in practice the detailed characteristics are not relevant to this invention. However, the means for indicating which E-DCH parameters the UE should use following a successful E-RACH access have not yet been decided.
For convenience we can refer to the mechanism for gaining uplink access using the existing RACH as “R99 RACH” and the new scheme as “E-RACH”.
The resources from which the UE is permitted to make a random selection for the R99 RACH preamble signal transmission are broadcast on a broadcast channel (BCH) in each cell. The R99 RACH parameters which are signalled on the BCH are included for reference in the table below. This information is provided for one of more PRACHs (Physical RACHs):
InformationElement/GroupTypenameNeedMultiand referenceSemantics descriptionVersion>>AvailableMPBitEach bit indicatesSignaturestring(16)availability for a signature,where the signatures arenumbered “signature 0” upto “signature 15”. The value1 of a bit indicates that thecorresponding signature isavailable and the value 0that it is not available.>>PreambleMPIntegerIdentification of scramblingscrambling code(0 . . . 15)code]number>>AvailableMPBit stringEach bit indicatesSub Channel(12)availability for aNumbersubchannel, where thesubchannels are numbered“subchannel 0” to“subchannel 11”. The value1 of a bit indicates that thecorresponding subchannel isavailable and the value 0indicates that it is notavailable.
The parameters for E-RACH access are likely to be defined in a similar way.
One scheme for indicating the E-DCH resources (for the data transmission which follows the E-RACH preamble transmission) which has been proposed in R1-074303, “Resource assignment for E-DCH access in CELL_FACH state” Nokia Corporation, Nokia Siemens Networks, available at http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_50b/Docs/R1-074303.zip, creates a link between the values of each of a group of multiple signatures sent on the AICH and a specific set of E-DCH parameters defined in a table, as can be seen below. Example of signature combinations from R1-074303, “Resource assignment for E-DCH access in CELL_FACH state”:
2 PRACH signatures and 6 E-DCH1 PRACH signaturesresources - two associated AICHand 7 E-DCH resources -signatures per PRACH (4 in total)three associated AICH signaturesUEUEtransmittedtransmittedPRACHNode BE-DCHPRACHNode BE-DCHpreambletransmittedresourcepreambletransmittedresourcesignatureAICH signaturesindexsignatureAICH signaturesindex11, 2 (−1, −1)NACK11, 2, 3 (−1, −1, −1)NACK11, 2 (−1, +1)111, 2, 3 (−1, −1, +1)111, 2 (+1, −1)211, 2, 3 (−1, +1, −1)211, 2 (+1, +1)311, 2, 3 (−1, +1, +1)323, 4 (−1, −1)NACK11, 2, 3 (+1, −1, −1)423, 4 (−1, +1)411, 2, 3 (+1, −1, +1)523, 4 (+1, −1)511, 2, 3 (+1, +1, −1)623, 4 (+1, +1)611, 2, 3 (+1, +1, +1)7
A number of problems exist with the solution proposed in R1-074303, “Resource assignment for E-DCH access in CELL_FACH state”:
if the AICH signature space is not increased, it reduces the number of preambles available, as multiple AICH signatures are mapped to each preamble. This is likely to increases the collision probability for both R99 RACH and E-RACH.—the UE needs to detect multiple signatures.
There could be an impact on legacy UEs since the detection probability of acknowledgements for R99 RACH would be degraded by multiple signature transmissions.
A possible solution, also identified in R1-074303, “Resource assignment for E-DCH access in CELL_FACH state” is to increase the number of AICH signatures (up to 32). This has the disadvantage that the detection probability for legacy UEs of acknowledgements for R99 RACH would be degraded by multiple (new) signature transmissions.
Another solution would be to also use non-transmission of a signature to indicate a different resource. For example:
UEtransmittedNode BPRACHtransmittedpreambleAICHE-DCHsignaturesignaturesresource index11, 2 (−1, 0)NACK11, 2 (−1, −1)111, 2 (−1, +1)211, 2 (+1, 0)311, 2 (+1, −1)411, 2 (+1, +1)5
This has the disadvantage of requiring more accurate signature amplitude detection at the UE.
As also mentioned in R1-074303, “Resource assignment for E-DCH access in CELL_FACH state”, the E-DCH resource could be indicated by an independent set of signatures. In R1-074303, it is suggested that this would allow only one simultaneous acknowledgement on the AICH. However, this restriction would not apply if the limitation can be accepted that resource allocation would not be independent for two simultaneous acknowledgements.
For example in the following case:
UEtransmittedNode BPRACHtransmittedpreambleAICHE-DCHsignaturesignaturesresource index11, 3 (−1, −1)NACK11, 3 (−1, +1)111, 3 (+1, −1)211, 3 (+1, +1)322, 3 (−1, −1)NACK22, 3 (−1, +1)422, 3 (+1, −1)522, 3 (+1, +1)6
If preamble signature 1 is acknowledged with 1,3 (+1,+1), then preamble signature 2 can be acknowledged with 2,3 (−1,+1) indicating resource 4 or with 2,3 (+1,+1) indicating resource 6 (but NACK or resource 5 cannot be indicated).
To summarize, in UMTS WCDMA a new RACH-based uplink is being discussed. The request for uplink transmission resource is made using a mechanism based on the conventional RACH. The main difference from the conventional RACH is that the data transmission uses a high-speed uplink packet data channel, known as Enhanced Dedicated Channel (E-DCH), instead of a single short message associated directly with the RACH preamble. One proposed scheme for indicating the E-DCH resources to be used creates a link between the values of each of a group of multiple signatures sent on the AICH (to acknowledge the E-RACH access) and a specific set of E-DCH parameters defined in a table.